1. Field of the Invention
The present invention relates generally to an oscillator for generating frequencies ranging over a wide-band and, more particularly to a variable oscillation frequency resonance circuit and voltage controlled oscillator using the same, which increases the quality factor of the resonance circuit and improves phase noise characteristics by reducing the impedance of switching elements at the time of selecting an oscillation band.
2. Description of the Prior Art
Generally, an oscillator generates a signal with a specific frequency in an electrical circuit. As shown in FIG. 1, an oscillator is generally comprised of a resonance circuit 11 for resonating with a signal with a specific frequency, and an amplifying circuit 12 for continuously supplying the signal with the specific frequency resonated by the resonance circuit 11. Further, the resonance circuit for determining the specific frequency is implemented by an LC circuit in which an inductance element (coil) and a capacitance element (capacitor) of electrical circuit elements are combined. In this case, the oscillation frequency is determined by the inductance L and the capacitance C of the resonance circuit, so it can be varied by changing the inductance L or capacitance C of the resonance circuit. For example, the oscillation frequency is varied by varying capacitance using a variable capacitance element, such as a varactor diode, and a variable capacitance range of the varactor diode corresponds to a variable frequency range.
A narrow-band oscillator can be implemented by only a variable capacitance element, such as the varactor diode. However, a receiver of a television (TV) system for receiving airwave broadcasting signals, or other multi-band receivers require wide-band oscillators capable of varying frequencies over a wide-band.
FIG. 2 is a circuit diagram showing an example of a variable oscillation frequency resonance circuit provided in the wide-band oscillator which can be used in the receiver of the TV system or the multi-band receivers. In this case, the resonance circuit is constructed in such a way that an inductance element L21, a fine frequency adjusting unit 21 consisting of two varactor diodes VD21 and VD22, connected in series with each other and having capacitances varied in response to a control signal Vctrl, and a band selecting unit 20 consisting of a plurality of capacitance elements and a plurality of switching elements and having a capacitance varied by ON/OFF operations of the plural switching elements performed in response to externally applied switching control signals VSW1 to VSWn, are connected in parallel with each other between output terminals A and B connected to the amplifying circuit 12.
In the resonance circuit, a resonance frequency (oscillation frequency) is determined by the sum of capacitances of the two varactor diodes determined in response to the control signal Vctrl and the capacitance of the band selecting unit 20 determined in response to the switching control signals VSW1 to VSWn, and the inductance of the inductance element L21.
More specifically, each of the varactor diodes VD21 and VD22 of the fine frequency adjusting unit 21 has an internal capacitance value CVAR linearly increased with the increase of a voltage level of the control signal Vctrl, as shown in FIG. 3a. That is, with the variation of the control signal Vctrl, the capacitance value of each of the varactor diodes VD21 and VD22 is continuously varied.
Further, the band selecting unit 20 is constructed in such a way that a plurality of capacitance elements are arranged in parallel with each other, and respective capacitance elements are connected in parallel with the output terminals A and B through the plural switching elements, thus switching capacitance elements connected to the output terminals A and B through the ON/OFF operations of the switching elements. Therefore, in the band selecting unit 20, a capacitance value is determined depending on capacitance elements connected to turned on switching elements. The capacitance value selected by the band selecting unit 20 is discontinuously varied in response to the switching control signals VSW1 to VSWn for controlling the ON/OFF operations of the switching elements, as shown in FIG. 3b. 
Accordingly, one of a plurality of oscillation bands as shown in FIG. 3b is selected by adjusting the switching control signals VSW1 to VSWn, and an oscillation frequency is determined within the selected oscillation band by adjusting the control signal Vctrl.
For example, if the number of cases of capacitance values selectable by the switching control signals VSW1 to VSWn of the band selecting unit 20 is four, as shown in FIG. 3b, one of four bands successively connected as shown in FIG. 3c is selected by the switching control signals VSW1 to VSWn, and the determination of a fine frequency within the selected band is carried out through the control of varactor diodes VD21 and VD22 using the control signal Vctrl.
Through the above construction of the resonance circuit, wide-band frequency oscillation necessary for a receiver of a TV system or other wide-band receivers is realized.
In this case, the construction of the conventional band selecting unit 20 is depicted in FIG. 4.
An example of a frequency varying circuit of the conventional band selecting unit 20 is described with reference to FIG. 4. The frequency varying circuit is constructed in such a way that drains of first to sixth switching transistors M41 to M46 are commonly connected to the output terminals A or B connected to the amplifying circuit 12, first to sixth capacitors C41 to C46 are connected between sources of the first to sixth transistors M41 to M46 and the ground, respectively, gates of the first to sixth transistors M41 to M46 are connected to each other in pairs, and first to third control signal input terminals VSW1 to VSW3 are connected to the connected three pairs of gates, respectively.
In the above construction, low or high level signals are applied to the first to third control signal input terminals VSW1 to VSW3, and ON/OFF characteristics of the first to sixth switching transistors M41 to M46 are determined by the low or high level signals. For example, provided that the first to sixth transistors M41 to M46 are NMOS transistors in the circuit of FIG. 4, a first-second switching transistor pair M41-M42, a third-fourth switching transistor pair M43-M44, and a fifth-sixth switching transistor pair M45-M46 are turned on if high level voltage signals are applied to the first to third control signal input terminals VSW1 to VSW3, while they are turned off if low level voltage signals are applied thereto. On the contrary, provided that the first to sixth switching transistors M41 to M46 are PMOS transistors, the above transistor pairs are turned on if low level voltage signals are applied to the first to third control signal input terminals VSW1 to VSW3, while they are turned off if high level voltage signals are applied thereto.
Further, the capacitor pairs C41-C42, C43-C44, and C45-C46 connected to the output terminals A and B through the switching transistors in FIG. 4 have different capacitances. A corresponding capacitor pair is connected to the inductance element L21 through the output terminals A and B to form a resonance circuit when corresponding transistors M41 to M46 connected to the control signal input terminals VSW1 to VSW3 are turned on.
Therefore, values applied to the first to third control signal input terminals VSW1 to VSW3 are varied, thus enabling the total number of cases of capacitance values connected to the output terminals A and B to be varied up to seven.
As described above, a capacitance value determined by the variation of control values applied to the first to third control signal input terminals VSW1 to VSW3 is connected to the varactor diodes VD21 and VD22 of FIG. 2, thereby determining the frequency of the resonance circuit 11.
However, the switching transistors M41 to M46 each have a predetermined resistance when they are turned on. Moreover, especially in the construction of FIG. 4, the switching transistors M41 to M46 are connected in series with the capacitors C41 to C46, respectively, so the quality factor Q of the resonance circuit unit is reduced due to the sum of resistances of the switching transistors M41 to M46.                     Q        =                              Im            ⁡                          (              z              )                                            Re            ⁢                                        z                                                                        [        1        ]            
Specifically, as indicated in Equation [1], the quality factor Q is represented by the ratio of reactance Im(z) to resistance Re|z| in the resonance circuit. As the quality factor Q increases, phase noise characteristics are improved.
However, in the conventional construction as shown in FIG. 4, two switching transistors connected as one pair on the basis of the output terminals A and B are connected in series with respective capacitors, so the total resistance Re|z| is increased to deteriorate the quality factor Q. Further, phase noise characteristics may be deteriorated due to the deterioration of the quality factor Q.